Common

When referring to items and drop rates, it usually means an item or quest update that drops every time or almost every time. Common items are of very little value.

When referring to a Language, Common is the standard word for the Common Tongue, that is, the language that all intelligent races and monsters in a given game can communicate in. Of course, not all NPCs or Monsters will speak Common, and you may need to learn their language to converse with them.